DIY: Replace the thermostat on a F-150
1. Remove the bolts that hold the thermostat intake cover onto the intake manifold of the F-150 with an adjustable wrench. Place the bolts off to the side. They will be reused during the installation process. Pull the radiator hose the thermostat cover is connected to into the air and let the antifreeze flow back into the radiator.
2. Pull the thermostat out of the exposed hole in the intake manifold with your fingers. The thermostat is not bolted into place. It sits on a machined flange inside of the intake manifold.
3. Pull the old carbon thermostat gasket off of the intake manifold and discard it. Place a bead of RTV silicone on each side of the new thermostat and place it onto the intake manifold. Make sure to line the holes cut out of the gasket for the bolts up with the actual bolt holes on the intake manifold.
4. Place the new thermostat back into the intake manifold with the correct end facing out of the engine. The end will be marked as the top of the thermostat.
5. Place the thermostat cover back onto the intake manifold and bolt it into place. Start the engine and check for leaks. If you see any leaks tighten the cover down to remove them.